1# @ohos.util.Deque (Linear Container Deque) 2 3Double-ended queue (deque) is a sequence container implemented based on the queue data structure that follows the principles of First In First Out (FIFO) and Last In First Out (LIFO). It allows insertion and removal of elements at both the ends. **Deque** can dynamically adjust the capacity based on project requirements. It doubles the capacity each time. **Deque** differs from **[Queue](js-apis-queue.md)** and **[Vector](js-apis-vector.md)** mainly in the following aspects: 4 5**Queue** follows the principle of FIFO only and allows element removal at the front and insertion at the rear. 6 7**Vector** supports insertion and deletion of elements in between, as well as at both the ends. When compared with **Vector**, **Deque** is more efficient in inserting and removing header elements, but less efficient in accessing elements. 8 9**Recommended use case**: Use **Deque** when you need to frequently insert or remove elements at both the ends of a container. 10 11This topic uses the following to identify the use of generics: 12- T: Type 13 14> **NOTE** 15> 16> The initial APIs of this module are supported since API version 8. Newly added APIs will be marked with a superscript to indicate their earliest API version. 17 18 19## Modules to Import 20 21```ts 22import Deque from '@ohos.util.Deque'; 23``` 24 25## Deque 26 27### Attributes 28 29**System capability**: SystemCapability.Utils.Lang 30 31| Name| Type| Readable| Writable| Description| 32| -------- | -------- | -------- | -------- | -------- | 33| length | number | Yes| No| Number of elements in a deque (called container later).| 34 35### constructor 36 37constructor() 38 39A constructor used to create a **Deque** instance. 40 41**System capability**: SystemCapability.Utils.Lang 42 43**Error codes** 44 45For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 46 47| ID| Error Message| 48| -------- | -------- | 49| 10200012 | The Deque's constructor cannot be directly invoked. | 50 51**Example** 52 53```ts 54let deque = new Deque(); 55``` 56 57### insertFront 58 59insertFront(element: T): void 60 61Inserts an element at the front of this container. 62 63**System capability**: SystemCapability.Utils.Lang 64 65**Parameters** 66 67| Name| Type| Mandatory| Description| 68| -------- | -------- | -------- | -------- | 69| element | T | Yes| Target element.| 70 71**Error codes** 72 73For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 74 75| ID| Error Message| 76| -------- | -------- | 77| 10200011 | The insertFront method cannot be bound. | 78 79**Example** 80 81```ts 82let deque = new Deque(); 83deque.insertFront("a"); 84deque.insertFront(1); 85let b = [1, 2, 3]; 86deque.insertFront(b); 87let c = {name : "Dylon", age : "13"}; 88deque.insertFront(c); 89deque.insertFront(false); 90``` 91 92### insertEnd 93 94insertEnd(element: T): void 95 96Inserts an element at the end of this container. 97 98**System capability**: SystemCapability.Utils.Lang 99 100**Parameters** 101 102| Name| Type| Mandatory| Description| 103| -------- | -------- | -------- | -------- | 104| element | T | Yes| Target element.| 105 106**Error codes** 107 108For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 109 110| ID| Error Message| 111| -------- | -------- | 112| 10200011 | The insertEnd method cannot be bound. | 113 114**Example** 115 116```ts 117let deque = new Deque(); 118deque.insertEnd("a"); 119deque.insertEnd(1); 120let b = [1, 2, 3]; 121deque.insertEnd(b); 122let c = {name : "Dylon", age : "13"}; 123deque.insertEnd(c); 124deque.insertEnd(false); 125``` 126 127### has 128 129has(element: T): boolean 130 131Checks whether this container has the specified element. 132 133**System capability**: SystemCapability.Utils.Lang 134 135**Parameters** 136 137| Name| Type| Mandatory| Description| 138| -------- | -------- | -------- | -------- | 139| element | T | Yes| Target element.| 140 141**Return value** 142 143| Type| Description| 144| -------- | -------- | 145| boolean | Returns **true** if the specified element is contained; returns **false** otherwise.| 146 147**Error codes** 148 149For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 150 151| ID| Error Message| 152| -------- | -------- | 153| 10200011 | The has method cannot be bound. | 154 155**Example** 156 157```ts 158let deque = new Deque(); 159deque.insertFront("squirrel"); 160let result = deque.has("squirrel"); 161``` 162 163### popFirst 164 165popFirst(): T 166 167Removes the first element of this container. 168 169**System capability**: SystemCapability.Utils.Lang 170 171**Return value** 172 173| Type| Description| 174| -------- | -------- | 175| T | First element removed.| 176 177**Error codes** 178 179For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 180 181| ID| Error Message| 182| -------- | -------- | 183| 10200011 | The popFirst method cannot be bound. | 184 185**Example** 186 187```ts 188let deque = new Deque(); 189deque.insertFront(2); 190deque.insertFront(4); 191deque.insertEnd(5); 192deque.insertFront(2); 193deque.insertFront(4); 194let result = deque.popFirst(); 195``` 196 197### popLast 198 199popLast(): T 200 201Removes the last element of this container. 202 203**System capability**: SystemCapability.Utils.Lang 204 205**Return value** 206 207| Type| Description| 208| -------- | -------- | 209| T | Last element removed.| 210 211**Error codes** 212 213For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 214 215| ID| Error Message| 216| -------- | -------- | 217| 10200011 | The popLast method cannot be bound. | 218 219**Example** 220 221```ts 222let deque = new Deque(); 223deque.insertFront(2); 224deque.insertEnd(4); 225deque.insertFront(5); 226deque.insertFront(2); 227deque.insertFront(4); 228let result = deque.popLast(); 229``` 230 231### forEach 232 233forEach(callbackFn: (value: T, index?: number, deque?: Deque<T>) => void, 234thisArg?: Object): void 235 236Uses a callback to traverse the elements in this container and obtain their position indexes. 237 238**System capability**: SystemCapability.Utils.Lang 239 240**Parameters** 241 242| Name| Type| Mandatory| Description| 243| -------- | -------- | -------- | -------- | 244| callbackFn | function | Yes| Callback invoked to traverse the elements in the container.| 245| thisArg | Object | No| Value to use when the callback is invoked.| 246 247callbackfn 248 249| Name| Type| Mandatory| Description| 250| -------- | -------- | -------- | -------- | 251| value | T | Yes| Value of the element that is currently traversed.| 252| index | number | No| Position index of the element that is currently traversed.| 253| deque | Deque<T> | No| Instance that invokes the **forEach** method.| 254 255**Error codes** 256 257For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 258 259| ID| Error Message| 260| -------- | -------- | 261| 10200011 | The forEach method cannot be bound. | 262 263**Example** 264 265```ts 266let deque = new Deque(); 267deque.insertFront(2); 268deque.insertEnd(4); 269deque.insertFront(5); 270deque.insertEnd(4); 271deque.forEach((value, index) => { 272 console.log("value:" + value, "index:" + index); 273}); 274``` 275 276### getFirst 277 278getFirst(): T 279 280Obtains the first element of this container. 281 282**System capability**: SystemCapability.Utils.Lang 283 284**Return value** 285 286| Type| Description| 287| -------- | -------- | 288| T | Element obtained.| 289 290**Error codes** 291 292For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 293 294| ID| Error Message| 295| -------- | -------- | 296| 10200011 | The getFirst method cannot be bound. | 297 298**Example** 299 300```ts 301let deque = new Deque(); 302deque.insertEnd(2); 303deque.insertEnd(4); 304deque.insertFront(5); 305deque.insertFront(4); 306let result = deque.getFirst(); 307``` 308 309### getLast 310 311getLast(): T 312 313Obtains the last element of this container. 314 315**System capability**: SystemCapability.Utils.Lang 316 317**Return value** 318 319| Type| Description| 320| -------- | -------- | 321| T | Element obtained.| 322 323**Error codes** 324 325For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 326 327| ID| Error Message| 328| -------- | -------- | 329| 10200011 | The getLast method cannot be bound. | 330 331**Example** 332 333```ts 334let deque = new Deque(); 335deque.insertFront(2); 336deque.insertFront(4); 337deque.insertFront(5); 338deque.insertFront(4); 339let result = deque.getLast(); 340``` 341 342### [Symbol.iterator] 343 344[Symbol.iterator]\(): IterableIterator<T> 345 346Obtains an iterator, each item of which is a JavaScript object. 347 348**System capability**: SystemCapability.Utils.Lang 349 350**Return value** 351 352| Type| Description| 353| -------- | -------- | 354| IterableIterator<T> | Iterator obtained.| 355 356**Error codes** 357 358For details about the error codes, see [Utils Error Codes](../errorcodes/errorcode-utils.md). 359 360| ID| Error Message| 361| -------- | -------- | 362| 10200011 | The Symbol.iterator method cannot be bound. | 363 364**Example** 365```ts 366let deque = new Deque(); 367deque.insertFront(2); 368deque.insertFront(4); 369deque.insertFront(5); 370deque.insertFront(4); 371 372// Method 1: 373for (let item of deque) { 374 console.log("value:" + item); 375} 376 377// Method 2: 378let iter = deque[Symbol.iterator](); 379let temp = iter.next().value; 380while(temp != undefined) { 381 console.log("value:" + temp); 382 temp = iter.next().value; 383} 384``` 385